Where the hell did you hear that? Akuma first appeared in Super Street Fighter II Turbo as a hidden boss. You could also play him with a code. He is NO way in hell a combination of Ken and Ryu. He has mastered their fighting style though. If you really want an indepth history you should pick up Street Fighter Alpha the movie and check out the comic. Anyways Akuma killed Ken and Ryu's master. Akuma of course has given into the dark hadou. I'm tired so you do your homework and all of your questions will be answered.
basically akuma was one of three original people who learned the art "killing fist" or something though i think now it has been referred to by many names such as "shotokan" but as of recent times it's anauskten
anyway as i said before akuma was one of three original people that leanred this art the second person was akuma's friend Gokuma (i know the names wrong but it's something like that) who would subsequently go onto teach Ryu, Ken (and yes Dan and Guy for a little while) the last was of course their master
now they practiced for many years and the time came for them to learn the advanced moves of their fighting style such as things like Raging demon and the Hadoken
Ken and Ryu's master rejected some of these advanced moves as he beileved they were barbaric and were moves not of a fighter but of a killer and so he refused to learn some of them
but akuma who didn't think the same revelled in learning these moves and enjoyd his new found power. Because as he learnt these moves he became more in touch with his dark and murderous side and the dark hadou.
Now apparently akuma got himself into a killing match with his master whereby at the end of the fight he murdered his master
While in his murder frenzy he attacked Gokuma who managed to escape
Though Akuma swore to finish what he started
So many years later after he had taught Ken, Ryu, (Guy and Dan for a little while)
Akuma eventually found his old friend at the time Capcom states that only Ken was there
Both Gokuma and Akuma fought which subsequently led to Gokuma's death at the hands of Akuma
Ken tried to intervene but at that point he was no match for Akuma and was brushed to one side.
This is more or less the story surrounding Akuma though it obviously goes into more detail you can check the rest out if you like